Methods for preparing thin sheets from powder material



July 24, 1962 A. J. KLING 3,045,289

METHODS FOR PREPARING THIN SHEETS FROM POWDER MATERIAL Filed Oct. 2, 1957 AGENT l' POWOEI? GINDER WATER frvveniror: August J KIN-1g,

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United States Patent Ofiice EfiiSZdfi Faierited July 24, 1962 3,045,2s9 METHODS FOR PREPARlNG T SHEETS FROM POWDER MATERIAL August J. Kling, 35 Glen Ava, Scotia, N.Y. Filed Oct. 2, 1957, Ser. No. 687,320 10 Claims. (Cl. Iii-57) The present invention relates to methods for preparing thin sheets from ceramic or other powder material, and more particularly it relates to methods forremoving such sheets from casting surfaces.

Powder material in thin sheet form is being used increasingly as, for example, for insulators in electron tubes. And thus improved methods for making these sheet are being sought.

Accordingly, an object of the present invention is to provide improved methods for making thin sheets from powder material.

Conventional casting methods have been used on powder materials, but these methods are successful only for the production of relatively thick sheets.

Thus, another object is to provide improved casting methods for producing relatively thin .sheets from powder material.

The principal difficulty in casting thin sheets from powder material by conventional methods is that the fragile sheets fracture as they are removed from the. casting surfaces.

Therefore, a further object of the present invention is to provide improved methods for removing thin sheets of powder material from casting surfaces.

These and other objects ,are achieved in one form of my invention by providing a rotating cylindrical surface upon which the desired sheet or film is cast, dried, and then subject to controlled rewetting prior to removal. The lower portion of the surface rotates in a solution or suspension containing the powder, a binder, and a liquid vehicle for the binder and powder so that a dip cast film coats the surface of the cylinder emerging from the solution or suspension. This film is then dried by an infrared lamp and subsequently sprayed with a wetting agent comprising a binder solvent suspended or mixed in controlled amounts in a vehicle that is relatively inert to the binder. Through use of this Wetting agent, resoftening of the film can be controlled so that, without disintegraation of the film, adhesion between the cylindrical surface and film is sufficiently reduced to permit removal of the film without fracture.

Although the novel features that I believe to be characteristic of my invention are set forth with particularity in the appended claims, my invention itself, however, can probably be best understood by reference to the following description taken in connection with the accompanying drawing in which the FIGURE illustrates one form of my invention.

Referring specifically to the figure, a cylinder 1, the width of which is approximately the same as the width of the desired sheet, has a surface, such as Plexiglas, that is sufficiently smooth to provide a casting surface. This cylinder is rotated by means here shown to be a motor 3, while partially immersed in a liquid t contained in a tank 5. Liquid 4 is a solution or suspension comprising the powder forthe sheet, a binder for the'powder, and a liquid vehicle for the binder and powder. The density of 2 liquid 4 can of course be controlled by the amount of powder and binder used. If the powder material is ceramic, one suitable solution for liquid 4 is a solution of water (the vehicle) and 2 to 3% by weight of polyvinyl alcohol (the binder) mixed with an equal volume of-powder.

As cylinder ll rotates, the surface tension between the cylinder surface and liquid 4 causes a film 6 to be coated over the cylinder surface as it emerges from the liquid. The film thickness can be controlled by the density of liquid 4, by the speed of rotation of cylinder 1, and also by the proximity of a doctor blade '7, or the like, placed adjacent to the cylinder surface.

Heating means, here shown to be an infra-red ray lamp 9, heats the film to a temperature such that the vehicle readily evaporates thereby leaving the powder and binder. If the film is thick, it can then be removed from the cylinder without further processing. But if it is thin,

the adhesive force between the film and cylinder surface may exceed the film strength so that the film is de stroyed upon removal unless this adhesive force is sufficiently reduced.

The preferred manner of reducing this force is by a controlled softening of the film that I call rewet-ting even though this term implies the use of water, which may or may not be used. A softener for the binder that is preferably a binder solvent, is employed to produce re wetting. The quantity of .this solvent must be controlled for if it is too much it softens the film to the extent ofdisintegration. And if it is too little it does not soften the film enough to sufficiently lower the adhesive force. The preferred manner of controlling the quantity of wetting agent is by mixing it in solution with a vehicle that is relatively inert to the binder so that once the film is saturated with the solution, the quantity ofwetting agent reaching the film. is constant. That is, as is characteristic of solutions, the wetting agent is in a state of equilibrium with the vehicle so that there is an equal tendency for the wetting agent to leave the film as to enter it. Thus, this solution can be applied to the filmwithout a consideration of the duration of application or of the quantity of solution, providing there is enough of both for saturation, because after saturation only a certain amount of wetting agent reaches the film.

Instead of being in solution the wetting agent can be suspended in the vehicle as for example in an emulsion. But then more precision may be required in adjusting the exposure of the film with respect to duration of application and quantity of suspension.

The illustrated manner of applying this rewetting liquid is through a hose or pipe it me nozzle 12 that sprays the liquid on the filrn thereby softening it to an extent that it can be removed from the cylinder surface without damage. polyvinyl alcohol, a water solvent of from 2 to 10% can be used in a vehicle of ordinary alcohol. In fact, commercial ethyl alcohol, which normally has approximately 5% water, may be adequate without the addition of water.

In those applications in which surface tension betweenthus lower the surface tension to an extent that it does if the binder is the aforementioned not cause breakage of the film when it is removed from the cylinder surface as a sheet 15.

In the production of very thin sheets, surface tension from the rewetting liquid may be enough to damage the film upon removal from the cylinder in spite of the use of a roller blotter 14. For the production of such sheets, a hand process is preferable.

Except for the last step, the hand process is similar to that mentioned. That is, a liquid comprising a solution or suspension of the powder, binder, and vehicle is spread on a casting plate in a film of suitable density and thickness and is either dried or allowed to dry on the plate. Then, and this is the materially different step, the film is rewet by immersion of the plate with film in a liquid containing a binder softener and inert liquid vc hicle. When so immersed there is, of course, no surface tension between the film and plate, and thus the film can be removed without tearing. If the film is very thin, it should be removed on a sheet of absorbent material such as filter paper and then blotted with fresh absorbent sheets to remove the excess liquid. Then the film can be allowed to dry under light pressure between absorbent sheets. Or, if it has been made suificiently dry by blotting, it can be pressed between two smooth flat surfaces.

The present invention may be applied to particular advantage to the manufacture of thin ceramic sheet insulating material useful, for example, in high frequency electric discharge devices. As a specific example, sheets of this type have been made by mixing together equal parts by volume of powdered A1 of 900 mesh and a 3% water solution of polyvinyl alcohol. The mixture is spread in a thin film on a Plexiglas casting plate. After the film is fully dry the film is rewet to a controlled degree. This wetting is accomplished by immersing the casting plate in ethyl alcohol containing about 5% by weight of water. The film is then peeled from the casting plate and dried.

Barium titanate, forsterite and other powdered ceramics have also been used. The 900 mesh powder is the finest ground crystalline A1 0 powder available. Mesh size as large as 500 mesh has been used in working .0005 mil sheets. Coarser powder may be used for thicker sheets. Also the amount of liquid used may be varied to facilitate control of the film thickness. Other water soluble resins, such as methyl cellulose, may be used instead of the polyvinyl alcohol.

There are no definite limits as to the thicknesses of the sheets produced by the methods of the present invention. Sheets as thin as half a mil have been made readily and thinner sheets could easily be made by these methods. For sheets above 40 mils thickness, other methods such as pressing become increasingly competitive, but no definite upper limit is known.

Although the invention has been described with respect to certain specific methods, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit of the invention. I intend, therefore, by the appended claims, to cover all such modifications and changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method for preparing thin sheets from powdered refractory material, said method comprising the steps of: coating a casting surface with a film of a mixture of the powder, a binder for the powder, and a liquid vehicle for the binder and powder; drying this film to remove the liquid vehicle; rewetting this film with a solution of binder softener and a vehicle inert to the binder wherein the portion of softener in said solution is less than that elfective to destroy the solidity of the film by saturation with said solution so that the force of adhesion between the film and casting surface is reduced to less than the strength of the film; and removing the film from the casting surface.

2. A method for preparing thin sheets from powder material, said method comprising the steps of: forming a film on a casting surface by coating said casting surface with a suspension of the powder, a binder for the powder, and a liquid vehicle for the binder and powder; removing the liquid vehicle from the film; rewetting the film with a liquid comprising a softener for the binder in solution with a vehicle that is inert to the binder wherein the percentage of binder softener is sufiicient to produce softening but not disintegration of said film by saturation of the film with the liquid; and removing said film from said casting surface.

3. A method for removing a thin film formed from powder material and binder from a casting surface, said method comprising the steps of: wetting the film with a liquid comprising a softener for the film binder in solution with a vehicle that is inert to the binder wherein the percentage of binder softener is sufficient to produce softening but not disintegration of said film by saturation of the film with the liquid; and removing said film from said casting surface.

4. A method for removing a thin film formed from powder material and binder from a casting surface, said method comprising the steps of: wetting the film with a liquid comprising a softener for the film binder in suspension with a vehicle that is inert to the binder wherein the percentage of binder softener is suflicient to produce softening but not disintegration of said film by saturation of the film with the liquid; and removing said film from said casting surface.

5. A method for removing a thin film formed from powder material and binder from a casting surface, said method comprising the steps of: immersing the casting surface with film in a liquid comprising a softener for the film binder in solution with a vehicle that is relatively inert to the binder wherein the percentage of binder softener is sufficient to produce softening but not disintegration of said film by saturation of the film with the liquid; removing the film from the casting surface and placing it on a piece of absorbent material; removing said absorbent material with film from said liquid; and drying said film by blotting with absorbent material.

6. A method for removing a thin film formed from powder material and binder from a casting surface, said method comprising the steps of: immersing the casting surface with film in a liquid comprising a softener for the film binder in suspension with a vehicle that is relatively inert to the binder wherein the percentage of binder softener is sufficient to produce softening but not disintegration of said film by saturation of the film with the liquid; removing the film from the casting surface; placing the film on a piece of absorbent material; removing said absorbent material with film from said liquid; and drying said film by blotting with absorbent material.

7. A method for preparing a thin sheet formed from powder material, said method comprising the steps of: rotating a cylindrical casting surface partially immersed in a liquid comprising a solution of the powder, a binder for the powder, and a liquid vehicle for the binder and powder, so that a film of said liquid is formed upon said casting surface as it emerges from said liquid; drying said film to remove the liquid vehicle therefrom; spraying a rewetting liquid on the dried film, wherein said rewetting liquid comprises a binder softener in solution in a vehicle inert to the binder and wherein the percentage of softener is great enough to soften the film but not great enough to disintegrate it by saturation of the film with the liquid; and then removing the film from said casting surface.

8. The method as defined in claim 7 including the step of blotting said film to remove the excess portion of rewetting liquid therefrom before removing said film from said casting surface.

9. A method for preparing a thin sheet from powder material, said method comprising the steps of: rotating a cylindrical casting surface partially immersed in a 3,045,289 5 6 liquid comprising a suspension of the powder, a binder for step of blotting said film to remove the excess portion of the powder and a liquid vehicle for the binder and powder, rewetting liquid therefrom before removing the film from so that a film of said liquid is formed upon said casting said casting surface.

surface as it emerges from said liquid; drying said film to remove the liquid vehicle therefrom; spraying a rewetting 5 References Cited in the file of this patent 0n the dried fi'lm, WileI'CiIl saiduewetting comprises a blnder softener 1n solution in a vehicle inert to the binder and wherein the percentage of softener is 2340550 Gflmmeter May 1936 great enough to soften said film but not great enough to 2,425,626 Llght 1947 disintegrate it by saturation of the film with the liquid; 1 25341113 Egger 1950 and then removing said film from said casting surface. 2:582993 Howatt 1952 10. The method as defined in claim 9 including the 2,747,583 Frankenburg at May 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,045,289 July 24, 1962 August Ja Kling It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the grant, lines 1 to3, for "August J. Kling, of Scotia, New York, read August J, Kling, of Scotia, New York, assignor to General Electric Company, a corporation of New York, line 12, for "August Jo Kling, his heirs" read General Electric Company, its successors in the heading to the printed specification, line 4, for "August J. Kling, 35 Glen Ave. Scotia, N. Y." read August J, Kling, Scotia, N. Y assignor to General Electric Company, a corporation of New York u Signed and sealed this 18th day of December 1962,,

(SEAL) Attest:

ERNEST w. SWIDER AVID L. LADD At st ng Offi Commissioner of Patents 

